Ageing or aging is the process of becoming older. This traditional definition was recently challenged in the new "Handbook of the Biology of Aging" (Academic Press, 2006), where ageing was specifically defined as the process of system's deterioration with time, thus allowing for existence of non-ageing systems (when "old is as good as new"), and anti-ageing interventions (when accumulated damage is repaired). This article focuses on the social, cognitive, cultural, and economic effects of ageing. The biology of ageing is treated in detail in senescence.
Ageing is an important part of all human societies reflecting the biological changes that occur, but also reflecting cultural and societal conventions.

Age is usually measured in full years (except for young children, where this downward rounding would be too crude) and a person's birthday is often an important event.

There is often considerable social pressure in many societies to sustain denial of the ageing process. Considerable energy, money and time is expended to hide signs of ageing, especially among women. This may involve dyeing hair, elaborate make-up, or even cosmetic surgery. Among the young, however, there is often a desire to seem older to gain more responsibility and respect.

The issues of an ageing population in which the average age of a society is increasing is an important issue in many nations of the world. The societal effects of age are great. Young people tend to commit most crimes, they are more likely to push for political and social change, to develop and adopt new technologies, and to need education. Older people have different requirements from society and government as opposed to young people, and frequently differing values as well. Older people are also far more likely to vote, and in many countries the young are forbidden from voting, and thus the aged have comparatively more political influence.

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Senescence: the biology of ageing

In biology, senescence is the state or process of ageing.
Cellular senescence is a phenomenon where isolated cells demonstrate a limited ability to divide in culture. Organismal senescence is the ageing of organisms.

Ageing is believed to have evolved because of the increasingly smaller probability of an organism still being alive at older age, due to predation and accidents, both of which may be entirely random and age-invariant. It is thought that strategies which result in a higher reproductive rate at a young age, but shorter overall lifespan, result in a higher lifetime reproductive success and are therefore favoured by natural selection.

Organismal ageing is generally characterized by the declining ability to respond to stress, increasing homeostatic imbalance and increased risk of disease. Because of this, death is the ultimate consequence of ageing.
Some researchers are treating ageing as a "disease" in gerontology (specifically biogerontologists), although this view is controversial. In scientific circles, the controversy of treating ageing as a disease is decreasing. That is, as genes that have an effect on ageing are discovered, ageing is increasingly being regarded in a similar fashion to other genetic conditions, including genetic diseases. For example, the sirtuin family of genes have been shown to have a significant effect on the lifespan of yeast and nematodes. Similarly, the discovery of genes for Tay-Sachs disease and insulin production reveal that many human conditions are genetically based. At the cell level, it has been shown that cells can be immortalized by the introduction of an additional gene for telomerase. In cell biology, there are links to deceleration of ageing by the presence of nutritionally derived polyphenol antioxidants.

Hans Baldung Grien: The Ages And Death, c. 1540-1543

Neurobiology of aging

Dividing the lifespan

A human life is often arbitrarily divided into various ages. Because biological changes are slow moving and vary from person to person arbitrary dates are usually set to mark periods of human life.
In some cultures the divisions given below are quite varied.

In the USA and other countries, adulthood legally begins at the age of eighteen or nineteen, while old age is considered to begin at age sixty-five.

Economics and marketing of ageing

The economics of ageing are also of great import. Children and teenagers have little money of their own, but most of it is available for buying consumer goods. They also have considerable impact on how their parents spend money.

Young adults are an even more valuable cohort. They often have jobs with few responsibilities such as a mortgage or children. They do not yet have set buying habits and are more open to new products.

The young are thus the central target of marketers. Television is programmed to attract the 15 to 35 years olds. Movies are also built around appealing to the young.

The middle aged and the old are less likely to buy things and are traditionally viewed as being set in their buying habits and not nearly as open to marketing. Older people tend to be much wealthier and to save a much higher percentage of their income.

Some tax systems attempt to address these differences in age spending habits such as the concept of a lifetime income tax.

Cultural variations

Considerable numbers of cultures have less of a problem with age compared with what has been described above, and it is seen as an important status to reach stages in life, rather than defined numerical ages. Advanced age is given more respect and status.

Cognitive changes in ageing

Steady decline in many cognitive processes are seen across the lifespan, starting in one's thirties. Research has focused in particular on memory and ageing, and has found decline in many types of memory with ageing, but not in semantic memory or general knowledge such as vocabulary definitions, which typically increases or remains steady.

Emotional improvements in aging

Given the physical and cognitive declines seen in ageing, a surprising finding is that emotional experience improves with age. Older adults are better at regulating their emotions and experience negative affect less frequently than younger adults and show a positivity effect in their attention and memory. The emotional improvements show up in longitudinal studies as well as in cross-sectional studies, and so cannot be entirely due to only the happier individuals surviving.

Global aging trends

There have been small changes in age distribution between 1990 and 2000.
The percentage of the population that is older increased slightly between 1990 and 2000 from 9% to almost 10%. The increase was larger within more developed countries, from 17.7% to 19.4%. The percentage of the population that is older is almost three times as high in more developed countries (19.4%) as it is in less developed countries (7.7%).